The present invention relates to full-wave rectifier circuits, and, more particularly, to a full-wave rectifier having a unity magnitude slope close to the origin.
Rectifiers are the fundamental building blocks in DC power supplies of all types and in DC power transmission used by some electric utilities. Specifically, full-wave rectifiers are often used in analog circuits for power detection of a received or transmitted signal. A single-phase full-wave rectifier circuit, shown in FIG. 1a, with the accompanying input and output voltage waveforms (FIGS. 1b and 1c, respectively) includes a center tapped transformer T1 coupled to a pair of diodes D1 and D2, wherein each diode conducts on opposite half-cycles of the input voltage.
As shown in FIG. 1c, while diode D1 conducts the first half-cycle of the input signal shown in FIG. 1b, diode D2 is off. During the second half-cycle, diode D2 conducts while diode D1 is off. The circuit changes a sinusoidal waveform with no dc component (zero average value) to one with a dc component of 2Vpeak/xcfx80, where the root mean square (rms) value of the output is 0.707Vpeak. This implementation is not preferred in an integrated circuit (IC) form since it is difficult to implement transformers in an IC. Further the use of diodes as shown has an electrical problem since the stage that drives the diodes can get severely loaded by the diodes and may need to provide high amounts of current.
Another implementation of the single-phase full-wave rectifier circuit, shown in FIG. 2a, may include a differential amplifier pair of transistors in lieu of the diode pair. Differential signals VB+Vi and VBxe2x88x92Vi are applied at the base of the two transistors Q1 and Q2, where VB is the bias voltage and Vi is the input voltage. The full-wave rectified voltage signal Vo is observed at the common emitter nodes of the two devices Q1 and Q2. An approximate transfer characteristic is shown in FIG. 2b. For bipolar devices that follow an exponential Ic vs. Vgs relationship, the output voltage Vo is represented by:
Voxcex1In(sech(xcex8i/2VT)
where VT is the thermal voltage which is equivalent to the Boltzmann constant, k, multiplied by the temperature, T, divided by the charge, q (kT/q).
The current approach suffers from reduced accuracy for small amplitudes of the signal. Specifically, this circuit has a dead zone close to its zero crossing. An ideal transfer function of the full-wave rectifier circuit is shown in FIG. 4a. A practical realizable transfer function of the circuit of FIG. 2a is shown in FIG. 4b. The dead zone near the zero crossing leads to the appearance of an error voltage ei, in response to a sinusoidal input as shown in FIG. 3. The effect of the dead zone is that the DC voltage output, for small amplitude inputs is much smaller compared to the ideal case. Mathematically, the unity magnitude slope for the implementation of FIG. 2a is approached only when xcex8i greater than  greater than VT, in which case sech(xcex8i/2VT)xcex1 exp(xe2x88x92|xcex8i|/2VT). Thus, Voxcex1xe2x88x92|xcex8i|, which has a slope of unity magnitude.
The non-unity slope near the zero-crossing causes problems in the rectification of very small signals, where xcex8i less than 2VT, as shown in FIGS. 4a, 4b and 4c. The output voltage of the rectifier is very much smaller than the ideal case.
One approach to solve this problem is to use amplification before the rectifier, but this requires increased power dissipation and reduces the upper limit of the dynamic range. The dynamic range is reduced by a factor of the reciprocal of the amplification. Further, the pre-amplifier needs to be linear over the range of input signals applied.
For example, where the amplification is 10 and the signals to be rectified have peak to peak excursions of 0.3 volt, the full-wave rectifier circuit would require 3 volts to operate. This presently is difficult in an IC implementation. Thus, there is a dynamic range tradeoff in which it is possible to rectify a signal from a smaller voltage input but it is not possible for larger voltages.
Thus, a need exists for an accurate full-wave rectification circuit having a unity magnitude slope close to the origin.
To address the above-discussed deficiencies of the biasing circuitry for single-ended circuits, the present invention teaches a full-wave rectifier having a unity magnitude slope close to the origin. In particular, a full-wave rectifier in accordance with the present invention includes an emitter coupled pair circuit coupled to a bias circuit. At least one constant current source couples to the base of each transistor in the emitter coupled pair circuit. A pair of transistors cross-couple across the emitter coupled pair circuit. These cross-coupled devices are used as positive feedback to increase gain for small amplitude signals and to degenerate the devices of the full-wave rectifier. Using this design very precise rectification can be achieved even for xcex8i less than VT.
Specifically, the bias circuit includes a current source which supplies xcex1 multiplied by the current supplied by the current source connected to the base of the transistors in the emitter coupled pair circuit. By choosing an appropriate value of xcex1, a unity magnitude slope close to the origin is achieved.